A drilling mud or fluid is typically pumped through a hollow drill stem and out an attached drill bit as an aid to drill rotation during the formation of a bore hole. In addition to rheological benefits, the drilling mud cools and lubricates the bit and stem, suspends drilling cuttings, and deposits a filter cake on the walls of the bore hole while transporting drilling cuttings upwardly to a settling pit on the surface. Removal of drilling cuttings from the borehole during drilling operations is essential. Drilling muds also typically function to prevent gases in a geological formation from escaping through bore hole walls, maintain bore hole stability, and protect formation productivity by preventing fluid loss.
Drilling muds may be characterized as: air-gas systems, including foam muds and aerated muds; oil-based systems, including invert-emulsion muds; and water-based systems, including bentonite muds, calcium muds, low-solids/nondispersed polymer muds, and salt muds.
Drilling muds contain a variety of components specifically selected for the depth, bore hole diameter, formation pressure, and structure of a particular well. These components generally include viscosifiers to enhance viscosity; fluid loss additives to prevent fluid penetration into formation reservoirs; weighting agents to consolidate drilling fragments and control formation pressures; dispersants to disperse solid matter; lost circulation materials to plug leaks in bore holes; shale controllers to prevent shale hydration and disintegration; emulsifiers and demulsifiers to improve bit efficiency; oil wetting agents and secondary emulsifiers to enhance fluid stability; and inorganic chemicals to adjust pH and supply proper ion concentrations. Other drilling mud components include torque reducing lubricants, foaming and defoaming agents, biocides, and corrosion inhibitors. A more detailed description of drilling muds can be found in Drilling Fluids Optimization: A Practical Field Approach by James L. Lummus and J. J. Azar.
Rheological control agents, and specifically viscosifiers, are among the most important drilling mud components because such agents permit the muds to function under diverse shear conditions. Drilling muds, for example, should ideally have low viscosity during mixing and pumping to minimize the energy input required during these operations. However, the viscosity should be sufficiently high during drilling so that drilling cuttings are maintained in suspension and carried out of the bore hole. Moreover, drilling muds must have sufficient gel strength to maintain cutting fragments in suspension during periods when drilling and pumping operations are stopped or the wellbore is highly deviated.
The hydrocarbon recovery industry preferably employs drilling muds that exhibit reduced viscosity as shear conditions increase. The relatively higher viscosity exhibited at lower shear conditions helps to maintain drilling cuttings in suspension toward the top of a bore hole, while lower viscosity exhibited under higher shear conditions improves drill bit lubrication, drilling mud flow rate, and drilling cutting distribution. High temperatures generated in proximity to the rotary drill bit also influence the rheological characteristics of drilling muds.
Drilling mud polymers are introduced is to provide enhanced viscosity and viscosity control, increased gel strength, and/or suspension and removal of drilling cuttings during drilling operations. Polymers used in drilling muds are usually based on water soluble derivatives of common polysaccharide materials such as xanthan, guar gum, other natural gums, cellulose ethers and esters, and bacterially produced water soluble polysaccharides. Xanthan is often used by the hydrocarbon recovery industry because it is very stable and functions as a fluid loss additive as well as a rheological control agent. Cellulose ethers and esters commonly employed as gellants include hydroxyethyl cellulose (HEC), carboxymethyl cellulose (CMC) carboxymethylhydroxyethyl cellulose (CMHEC), and hydroxypropylmethyl cellulose (HPMC). Additional polymers used include partially hydrolyzed polyacrylamides (PHPA), polyacrylamide/polyacrylate co-polymers, mixed metal hydroxides, and hydroxypropyl guar.
Well conditions, particularly well temperatures, have significant bearing on the choice of polymer. Drilling deeper wells that typically exhibit higher operating temperatures presents challenges and requires greater control over the rheological properties of drilling muds. Additionally, the composition of the make-up water can dramatically affect the properties of polymers used in drilling muds. Fresh, purified water is unavailable at many well sites. The presence of metal ions and varying salt concentrations may adversely affect how a particular polymer functions, particularly at high temperatures.
In general, increasing the polymer concentration in the drilling mud results in increased viscosity. Practical, economical, and operational considerations, however, limit the amount of polymer that can be introduced to a drilling mud to increase its viscosity. Additionally, excessive polymer loading may result in poor mixing efficiency and substantial frictional resistance.
The behavior of drilling muds is evaluated in terms of plastic viscosity, yield point, and gel strength. The plastic viscosity or shearing stress expresses the internal resistance to fluid flow resulting from the interaction of solids in the drilling mud. The yield point expresses the internal resistance of the mud to initial flow. The gel strength expresses the electrical attractive forces within the drilling mud under static conditions.
Although substantial research efforts have been devoted to developing stable and economically feasible drilling muds that exhibit the desired plastic viscosity, yield point, gel strength, and other rheological properties, the results have not been entirely satisfactory. The present invention is therefore directed to providing economically feasible drilling mud additives to produce drilling mud compositions that provide improved rheological properties and are stable throughout drilling operations.